Add automatic asset optimization to build process

[FeralAI]

While updating the frontend dependencies in #31 I did a manual pass at optimizing the favicon and header image. As the web UI gets more complex I forsee more image assets being added, and believe a good step to keeping the app size down would be to add a build step to run assets through an image optimizer. I've worked with https://www.npmjs.com/package/imagemin in the past and it works great, but there's probably some discovery to figure out what tool is best.